In an optical communication field or an optical measurement field, an optical waveguide device in which an optical waveguide or modulation electrodes are formed on a substrate having an electro-optical effect or the like has been often used in the related art.
In particular, since the amount of transmitted information tends to increase with the development of multimedia, it is necessary to widen a band of a light modulation frequency. In order to realize this, an external modulation method using an LN modulator or the like has been proposed. However, in order to widen the band of the LN modulator, it is necessary to realize speed matching between a light wave and a microwave, which is a modulated signal, and to reduce a driving voltage.
In order to solve the problems, it is known that a condition of speed matching between a microwave and a light wave is satisfied, and at the same time, a driving voltage is reduced by making a substrate thinner than in the related art.
In the following Patent Document 1 or 2, an effective refractive index of a microwave is reduced by providing an optical waveguide and modulation electrodes on a thin substrate (hereinafter, referred to as a ‘first substrate’) having a thickness of 30 μm or less and bonding another substrate (hereinafter, referred to as a ‘second substrate’) having a dielectric constant lower than the first substrate to the first substrate, such that the speed matching between the microwave and the light wave is realized and the mechanical strength of the substrate is maintained.    Patent Document 1: JP-A-64-18121    Patent Document 2: JP-A-2003-215519
In Patent Document 1 or 2, LiNbO3 (hereinafter, referred to as ‘LN’) is mainly used for the first substrate and a material having a lower dielectric constant than LN, such as quartz, glass, and alumina, is mainly used for the second substrate. In the combination of these materials, temperature drift or DC drift according to a temperature change occurs due to a difference between coefficients of linear expansion. In order to eliminate such problem, Patent Document 2 discloses that the first substrate and the second substrate are bonded to each other using an adhesive having a coefficient of linear expansion close to the first substrate.
On the other hand, use of the optical waveguide device is not limited to the light modulator described above. The optical waveguide device is requested to have multiple functions, for example, to be used as a diffraction grating or a variable wavelength filter. In addition, the optical waveguide device is also requested to have high performance, for example, to monitor a control state of the optical waveguide device or an optical integrated circuit, in which a plurality of optical waveguide devices are provided.
For example, in a variable wavelength filter, not only an optical waveguide or an electrode is provided in an optical waveguide device, but also a titanium oxide layer and unevenness need to be formed on the optical waveguide in order to form the refractive index distribution which periodically changes along the optical waveguide as disclosed in Patent Document 3 or 4.    Patent Document 3: JP-A-5-88123    Patent Document 4: JP-A-5-264809
Moreover, in the optical integrated circuit, optical waveguides need to be bent near an end of a substrate, and accordingly, a reflecting means, such as a green lens, needs to be arranged on the end surface of the substrate. In addition, in order to monitor a light wave propagating through the optical waveguide device, it is necessary to provide an optical waveguide or a directional coupler for monitoring.
Thus, in order to realize an optical waveguide device having multiple functions or high performance, the number of optical waveguides or electrodes provided in the optical waveguide device increases and various kinds of films, unevenness shape, and the like increase. As a result, the optical waveguide device itself becomes complicated. Furthermore, if the members described above are formed on the thin plate described above, the thin plate is easily damaged due to a difference between coefficients of thermal expansion of the thin plate and each of the members or due to mechanical or thermal stress distortion caused by the complicated shape of the thin plate. As a result, problems that yield of products decreases, unnecessary stress is applied to the thin plate and an operating characteristic of the optical waveguide device deteriorates, and the like occur.